The present invention relates generally to a super-conductive coil in a synchronous rotating machine. More particularly, the present invention relates to a coil support structure for super-conducting field windings in the rotor of a synchronous machine.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core super-conductive rotors require large amounts of super-conducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor.
High temperature SC coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K, to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
Super-conducting coils have been cooled by liquid helium. After passing through the windings of the rotor, the hot, used helium is returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power.
Prior SC coil cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. However, immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature. As a result, no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures.
What is needed is a super-conducting field winding assemblage for an electrical machine that does not have the disadvantages of the air-core and liquid-cooled super-conducting field winding assemblages of, for example, known super-conductive rotors.
In addition, high temperature super-conducting (HTS) coils are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start-up and shut-down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor winding should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature, and notwithstanding occasional over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the windings over normal operating conditions.
SC coils used as the HTS rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation. They are subjected to centrifugal loading, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the SC coils should be properly supported in the rotor by a coil support system. These coil support systems hold the SC coil(s) in the HTS rotor and secure the coils against the tremendous centrifugal forces due to the rotation of the rotor. Moreover, the coil support system protects the SC coils, and ensures that the coils do not prematurely crack, fatigue or otherwise break.
Developing support systems for HTS coils has been a difficult challenge in adapting SC coils to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a HTS rotor having a coil support system for a SC coil. The need also exists for a coil support system made with low cost and easy-to-fabricate components.
A coil support system has been developed for a racetrack shaped, high temperature super-conducting (HTS) coil winding for a two-pole rotor of an electrical machine. The coil support system prevents damage to the coil winding during rotor operation, supports the coil winding with respect to centrifugal and other forces, and provides a protective shield for the coil winding. The coil support system holds the coil winding with respect to the rotor. The HTS coil winding and coil support system are at cryogenic temperature while the rotor is at ambient temperature.
The split-housing coil support is particularly useful for a low power density High Temperature Super-conducting (HTS) electric machine. The coil support withstands the high centrifugal and tangential forces that would otherwise act on the SC coil. The coil housings are positioned end-to-end along the long side sections of the coil winding in order to evenly distribute the centrifugal and tangential forces that act on the coil. To reduce the heat leakage, the mass of the coil support has been minimized to reduce thermal conduction from the rotor through support into the cold coil. The coil support is maintained at cryogenic temperatures, as is the field winding.
The coil support system includes a series of coil support assemblies that span between opposite sides of the racetrack coil winding. Each coil support assembly includes a tension rod and a pair of split coil housings. The tension rods extend between opposite sides of the coil winding through conduits, e.g., holes, in the rotor core. A split coil housing at each end of the tension rod attaches to the coil. The housing transfers centrifugal forces from the coil to the tension rod. Each coil support assembly braces the coil winding with respect to the rotor core. The series of coil support assemblies provides a solid and protective support for the coil winding.
Each split coil housing comprises a pair of opposite side panels that are assembled around the SC coil and grasps an end of the tension rod. The side panels are xe2x80x9cCxe2x80x9d shape pieces which are fastened together by bolts to enclose the coil between a pair of side panels. Clamping bolts hold the side panels together and prevent the coil housing from splitting under large centrifugal and tangential loads.
The HTS rotor may be for a synchronous machine originally designed to include SC coils. Alternatively, the HTS rotor may replace a copper coil rotor in an existing electrical machine, such as in a conventional generator. The rotor and its SC coils are described here in the context of a generator, but the HTS coil rotor is also suitable for use in other synchronous machines.
The coil support system is useful in integrating the coil support system with the coil and rotor. In addition, the coil support system facilitates easy pre-assembly of the coil support system, coil and rotor core prior to final rotor assembly. Pre-assembly reduces coil and rotor assembly time, improves coil support quality, and reduces coil assembly variations.
In a first embodiment, the invention is a rotor for a synchronous machine comprising: a rotor core; a super-conducting coil winding extending around at least a portion of the rotor core, the coil winding having a side section adjacent a side of the rotor core; at least one tension rod extending through a conduit in the rotor core; and a housing attached to the tension rod and connected to the side section of the coil winding, wherein the housing comprises a pair of side panels.
In another embodiment, the invention is a method for supporting a super-conducting coil winding in the rotor core of a synchronous machine comprising the steps of: extending a tension rod through a conduit in the rotor core; positioning the coil winding around the rotor core such that the tension rod and tension bolt span between side sections of the coil winding; assembling a pair of side panels of at least one housing around a side section of the coil winding; securing side panels together, and attaching the housing to a first end of the tension rod.